1. Field of the Invention
The present invention relates to an apparatus and method for measuring a Depth-Of-Interaction (hereinafter referred to as a ‘DOI’) which is capable of improving the spatial resolution in a Positron Emission Tomography (hereinafter referred to as a ‘PET’). More particularly, the present invention relates to an apparatus and method for measuring a DOI using a light dispersion property within the crystal layer of a mono layer into which reflective films are inserted, and a PET using the same.
2. Background of the Related Art
The PET (Positron Emission Tomography) is a tomography using radioactive rays, such as X-ray Computerized Tomography (CT) tomography and a Single Photon Emission Computerized Tomography (SPECT).
The PET is typically a technique for imaging a distribution of foreign substances within the body by injecting a radioactive sample, emitting positrons, into an organism through an intravenous injection or inhalation and detecting the emitted positrons in order to conduct researches and diagnosis. This technique is the same principle as, for example, a technique of using FDG in which a radioactive isotope F-18 having a half life of about 110 minutes is combined with glucose in order to track cancer cells based on the fact that some cancer cells accumulate more glucose than other cells.
As described above, the PET is being used in metabolism researches on the human body, a diagnosis of cancer, and the diagnosis and researches for several diseases, such as the heart and nervous system abnormalities. A positron emission nuclide is an unstable isotope having some number of neutrons in a nucleus. Nuclides, such as O15, N13, C11, and F18, are mainly used in the PET.
Positrons emitted from the positron emission nuclides within the human body are combined with nearby electrons by a phenomenon called “pair annihilation,” thus emitting γ-rays.
In accordance with the principle of the conservation of energy and law of conservation of momentum related to the mass-energy equivalence principal, the positrons being in a static state are combined with nearby electrons and then converted into annihilation gamma rays of 511-keV energy, which are emitted in the opposite directions. A position where γ-rays are generated can be determined by detecting and analyzing a pair of γ-rays emitted in the opposite direction. Accordingly, the occurrence frequency of γ-rays, that is, the accumulated concentration of a marked sample can be found as a function of spatial position coordinates. A distribution of radioactive nuclides within the body of an examinee can be known by displaying the results using display means, etc.
The most important factors to determine the performance of the PET are the spatial resolution and detection efficiency. To achieve improved performance, a method of densely arranging detectors having a smaller size is possible. This method is, however, disadvantageous in that a reduction in the size of components is limited and the cost is increased because of an increase in the number of detectors and electronic measuring instruments.
As a method of improving the spatial resolution, there is a method using DOI information. The term ‘DOI’ refers to a depth from a crystal to a place where scintillation light is generated. If the DOI information is unknown, a PET apparatus will have a significant error in determining a position where gamma rays has been generated because of the parallax errors in the peripheral field of view, inevitably resulting in a degraded spatial resolution. Accordingly, to maintain a certain spatial resolution and detection efficiency without causing deterioration in spatial resolution uniformity, DOI information within the crystal is used.
One of the DOI estimation methods is a method based on that the temporal characteristic or size of a scintillation light signal emission differs in multiple crystal layers with different properties. In this method, multiple crystal layers are formed in a DOI measurement apparatus in order to measure the DOI. However, this method is disadvantageous in that it provides only discrete DOI information, which is limited by the number of layers. Light losses between the layers and expense versus mono-layer crystal designs are also drawbacks of this method.
Another method of the DOI estimation is to count the number of photons using photosensors attached to both ends of a scintillation light crystal in the length direction. In this method, the DOI is measured as a ratio of detections because a photosensor close to a DOI position detects a greater number of scintillation light signals. This method is, however, disadvantageous in that a lot of costs are required because the photosensors have to be provided on both sides of the scintillation light crystal.
To solve the problem, there was a proposed method of measuring light shared between crystals by installing the photosensor only on one face of the scintillation light crystal in the length direction and combining a reflective film on a surface of the crystal. This method is based on that the reflective film is partially inserted between the crystals and the amount of light shared between the crystals is changed according to the depth of the crystal.
In particular, in the method, a pair of two scintillation light crystals is used as one scintillation light detector unit, light is shared only between the pair of two scintillation light crystals, and the number of photons shared only between the two scintillation light crystals is compared. Accordingly, if it is sought to measure light using a photomultiplier tube (hereinafter referred to as a ‘PMT) of a multi-channel, it is indispensable to match the scintillation light crystal to each light pixel. If it is sought to measure light using photosensors, the photosensor must be attached to each crystal. This method is also problematic in that a DOI response is deteriorated because light is dispersed in all directions by glass between the light pixel and a surface of the PMT, and the manufacturing costs are increased because of an increase in the number of photosensors and of an increased size of electronic equipment accordingly. Accordingly, there has been a need for a model of a DOI measurement apparatus and a DOI measurement method using the same, which can solve the above-described problems.